Communication of data over computer networks, particularly the Internet, has become an important, if not essential, way for many organizations and individuals to disseminate information. The Internet is a global network connecting millions of computers using a client-server architecture in which any computer connected to the Internet can potentially receive data from and send data to any other computer connected to the Internet. The Internet provides a variety of methods in which to communicate data, one of the most ubiquitous of which is the World Wide Web, also referred to as the web. Other methods for communicating data over the Internet include e-mail, Usenet newsgroups, telnet and FTP.
The World Wide Web is a system of Internet servers, typically called “web servers”, that support the documents and applications present on the World Wide Web.
Documents, known as web pages, may be transferred across the Internet according to the Hypertext Transfer Protocol (“HTTP”) while applications may be run by a Java virtual machine present in an internet browser. Web pages are often organized into web sites that represent a site or location on the Web. The web pages within a web site can link to one or more web pages, files, or applications at the same web site or at other web sites. A user can access web pages using a browser program running on the user's computer or web-enabled device and can “click on” links in the web pages being viewed to access other web pages.
Each time the user clicks on a link, the browser program generates a request and communicates it to a web server hosting web pages or applications associated with the web site. The web server retrieves the requested web page or application from an application server or Java server and returns it to the browser program. Web pages and applications can provide a variety of content, including text, graphics, interactive gaming and audio and video content.
Because web pages and associated applications can display content and receive information from users, web sites have become popular for enabling commercial transactions. As web sites become more important to commerce, businesses are increasingly interested in quickly providing responses to user's requests. One way of accelerating responses to requests on a web site is to cache the web pages or applications delivered to the requesting user in order to allow faster access time to this content when it is next requested.
Commercial web sites typically want to serve different versions of a page to different requesters even though those requesters all request the same Uniform Resource Locator (URL). For example, the front page of a site is often addressed as /index.html or /index.jsp, but the site operator may wish to deliver different versions of that page depending upon some property of the requester. Common examples are versions of a page in different languages. The selection of an appropriate variant to serve is commonly known as content negotiation, which is defined in the Hypertext Transfer Protocol (HTTP) specification.
Existing content negotiation schemes (as typified in Request for Comments (RFCs) 2616, 2295, and 2296) apply to general characteristics of content: the language used in the content, the style of markup, etc. A user-agent (i.e., a client application used with a particular network protocol, particularly the World Wide Web) can include in a request a description of its capabilities and preferences in these areas, and a server can deduce the best version of content to send in response. For example, a client application may specify, via headers in an HTTP request, that it prefers to receive English, French, and German content, in that order; if the server receives a request for a page that is available only in French and German, it will send the French version in response. This preference will only be applied when there is a choice of representations which vary by language. It's also possible for the server to respond with a list of possible options with the expectation that the client application will then employ its own algorithm to select one of those options and request it. These schemes rely on a certain degree of cooperation on the client application's part, and concern variations that the client application can reasonably be expected to be aware of.
Currently, some servers support driven content negotiation as defined in the HTTP/1.1 specification. Some servers also support transparent content negotiation, which is an experimental negotiation protocol defined in RFC 2295 and RFC 2296. Some may offer support for feature negotiation as defined in these RFCs. An HTTP server like Apache provides access to representations of resource(s) within its namespace, with each representation in the form of a sequence of bytes with a defined media type, character set, encoding, etc. A resource is a conceptual entity identified by a URI (RFC 2396). Each resource may be associated with zero, one, or more than one representation at any given time. If multiple representations are available, the resource is referred to as negotiable and each of its representations is termed a variant. The ways in which the variants for a negotiable resource vary are called the dimensions of negotiation.
In order to negotiate a resource, a server typically needs to be given information about each of the variants. In an HTTP server, this can be done in one of two ways: consult a type map (e.g., a *.var file) which names the files containing the variants explicitly, or do a search, where the server does an implicit filename pattern match and chooses from among the results. In some cases, representations or variants of resource are stored in a cache. When a cache stores a representation, it associates it with the request URL. The next time that URL is requested, the cache can use the stored representation. However, if the resource is negotiable at the server, this might result in only the first requested variant being cached and subsequent cache hits might return the wrong response. To prevent this, the server can mark all responses that are returned after content negotiation as non-cacheable by the clients.